Photocatalytic RuO₂/TiO₂ nanohybrids have been engineered using flame spray pyrolysis (FSP) technology, enabling the selective formation of either {(rutile)TiO₂–(rutile)RuO₂} or {(anatase)TiO₂–(rutile)RuO₂} nanointerphase in a single step. The {(rutile)TiO₂–(rutile)RuO₂} nanohybrids were selectively produced by using a double-nozzle FSP (DN-FSP) process. The {(anatase)TiO₂–(rutile)RuO₂} nanohybrids were selectively produced using a single-nozzle FSP (SN-FSP) process. Both RuO₂/TiO₂ nanohybrids exhibited significant photocatalytic H₂ and O₂ production from H₂O. Notably, the {(rutile)TiO₂–(rutile)RuO₂} is shown to be a superior photocatalyst, achieving H₂ evolution rates exceeding 5144 μmol g^–1 h^–1 H₂ and 2369 μmol g^–1 h^–1 O₂ under solar light illumination. This performance is 50% higher than that of the {(anatase)TiO₂–(Rutile)RuO₂} configuration. In situ EPR spectroscopy reveals that stable Ti³⁺/Ru⁵⁺ centers are formed during the FSP process, in the {(rutile)TiO₂–(rutile)RuO₂} interphase. This highlights a unique interfacial electron-transfer event, which is ascribed to a strong oxide (RuO₂)/support (TiO₂) interaction (SOSI), further validated by transmission electron microscopy/scanning transmission electron microscopy. From a technological perspective, FSP fosters a robust {rutile–rutile} interface between the semimetal–RuO₂ and the TiO₂ semiconductor, which is beneficial for the separation of interfacial electron–hole pairs onto TiO₂ and RuO₂ particles. Double-nozzle FSP technology, as demonstrated in this study, offers a promising engineering approach for the industrial production of high-efficiency photocatalytic materials.

Nature special issue ”Hydrogen and alternative fuel sources”
ΜΕΝΤΙΟΝ ΤΟ OUR WORK
Control of monomeric Vo’s versus Vo clusters in ZrO_2−x for solar-light H₂ production from H₂O at high-yield (millimoles gr⁻¹ h⁻¹)

Abstract

Flame spray pyrolysis (FSP) is an industrially scalable technology that enables the engineering of a wide range of metal-based nanomaterials with tailored properties nanoparticles. In the present review, we discuss the recent state-of-the-art advances in FSP technology with regard to nanostructure engineering as well as the FSP reactor setup designs. The challenges of in situ incorporation of nanoparticles into complex functional arrays are reviewed, underscoring FSP’s transformative potential in next-generation nanodevice fabrication. Key areas of focus include the integration of FSP into the technology readiness level (TRL) for nanomaterials production, the FSP process design, and recent advancements in nanodevice development. With a comprehensive overview of engineering methodologies such as the oxygen-deficient process, double-nozzle configuration, and in situ coatings deposition, this review charts the trajectory of FSP from its foundational roots to its contemporary applications in intricate nanostructure and nanodevice synthesis.

Nanomaterials 2023, 13(23), 3006; https://doi.org/10.3390/nano13233006 

Abstract

Cu₂O is a highly potent photocatalyst, however photocorrosion stands as a key obstacle for its stability in photocatalytic technologies. Herein, we show that nanohybrids of Cu₂O/Cu⁰ nanoparticles interfaced with non-graphitized carbon (nGC) constitute a novel synthesis route towards stable Cu-photocatalysts with minimized photocorrosion. Using a Flame Spray Pyrolysis (FSP) process that allows synthesis of anoxic-Cu phases, we have developed in one-step a library of Cu₂O/Cu⁰ nanocatalysts interfaced with nGC, optimized for enhanced photocatalytic H₂ production from H₂O. Co-optimization of the nGC and the Cu₂O/Cu⁰ ratio is shown to be a key strategy for high H₂ production, > 4700 μmoles g⁻¹ h⁻¹ plus enhanced stability against photocorrosion, and onset potential of 0.234 V vs. RHE. After 4 repetitive reuses the catalyst is shown to lose less than 5% of its photocatalytic efficiency, while photocorrosion was < 6%. In contrast, interfacing of Cu₂O/Cu⁰ with graphitized-C is not as efficient. Raman, FT-IR and TGA data are analyzed to explain the undelaying structural functional mechanisms where the tight interfacing of nGC with the Cu₂O/Cu⁰ nanophases is the preferred configuration. The present findings can be useful for wider technological goals that demand low-cost engineering, high stability Cu-nanodevices, prepared with industrially scalable process.

Abstract

Plasmonic nanoparticles (PNPs) constitute a significant category of photoresponsive materials whose exploitation in photoboosted catalysis is a forward-looking strategy. Here, it is demonstrated that photoexcited core–shell Ag⁰@SiO₂ PNPs can dramatically enhance formic acid dehydrogenation (FADH), catalyzed by the molecular catalyst [Fe(BF₄)₂·6H₂O/P(CH₂CH₂PPh₂)₃, PP₃]. In the presence of photoexcited Ag⁰@SiO₂ PNPs, the optimized catalytic system [(Fe/PP₃)/HCOOH/Ag⁰@SiO₂/hv] achieves an almost 10-fold increase of the H₂-gas-production rate vs [(Fe/PP₃)/HCOOH] (173 vs 17 mL H₂ min^–1, using 12.5 μmol of catalyst), while the turnover numbers (TONs) are boosted by ∼400% (35,643 vs 9615) and the turnover frequencies (TOFs) by ∼600% (17,821 h^–1vs 2885 h^–1). Selective excitation at wavelengths (λ_ex) spanning the photoresponse profile of Ag⁰@SiO₂ NPs demonstrates that the FADH enhancement is maximal at λ_ex = 405 nm, which is at the peak of the photoplasmonic response of Ag⁰@SiO₂ NPs. Monitoring of the solution potential (E_h) under catalytic conditions reveals that the photoexcitation of Ag⁰@SiO₂ PNPs injects hot electrons, as reducing agents, into the reaction solution. Varying the SiO₂-shell thickness of Ag⁰@SiO₂ PNPs in the range of 3–5 nm allowed control of the hot-electron injection rates and the ensuing FADH rates. The present results are discussed in the context of the catalytic cycle of the [(Fe/PP₃)/HCOOH] system, where plasmonically generated hot electrons boost H₂ production via FADH by molecular catalysts, in distinction to the thermoplasmonic effects that seem to play a secondary role. The present H₂-production rate data demonstrate the possibility to approach industrial-scale H₂-production rates via FADH, using low-cost Fe-based catalysts and no sacrificial cocatalysts. We consider that the phenomenon exemplified herein for a standard molecular-catalysis system, such as [(Fe/PP₃)/HCOOH], can be valid for many other pertinent molecular FADH catalysts.

Scientific Reports article ”Control of monomeric Vo’s versus Vo clusters in ZrO_2−x for solar-light H₂ production from H₂O at high-yield (millimoles gr⁻¹ h⁻¹)” has been added to the collection ”Hydrogen and alternative fuel sources”

ABSTRACT

Pristine zirconia, ZrO2, possesses high premise as photocatalyst due to its conduction band energy edge. However, its high energy-gap is prohibitive for photoactivation by solar-light. Currently, it is unclear how solar-active zirconia can be designed to meet the requirements for high photocatalytic performance. Moreover, transferring this design to an industrial-scale process is a forward-looking route. Herein, we have developed a novel Flame Spray Pyrolysis process for generating solar-light active nano-ZrO2−x via engineering of lattice vacancies, Vo. Using solar photons, our optimal nanoZrO2−x can achieve milestone H2-production yield,> 2400 μmolg−1 ­h−1 (closest thus, so far, to high photocatalytic water splitting performance benchmarks). Visible light can be also exploited by nano-ZrO2−x at a high yield via a two-photon process. Control of monomeric Vo versus clusters of Vo’s is the key parameter toward Highly-Performing-Photocatalytic ZrO2−x. Thus, the reusable and sustainable ZrO2−x catalyst achieves so far unattainable solar activated photocatalysis, under large scale production.

Reversible Plasmonic Switch in a Molecular Oxidation Catalysis Process

Abstract

Currently, plasmonic nanoparticles (PNPs) are considered highly efficient enhancers of catalytic processes. Herein, we report a concept where plasmonic Ag⁰@SiO₂ nanoparticles can reversibly switch-off an oxidation catalytic process under light-excitation. The catalytic process recommences when illumination is stopped. The catalytic system under study is a well-characterized molecular LMn^II catalyst that performs alkene oxidation, with H₂O₂ as the oxidant. Three types of plasmonic core–shell Ag⁰@SiO₂ nanoparticles, with a SiO₂ shell of varying thickness (0.1–5 nm), were utilized in this study. Using electron paramagnetic resonance spectroscopy, we have identified the reversible inhibition of the transient LMn^IV═O intermediate formation, to be the key-step of the photoinduced pause of the catalytic process by the Ag⁰@SiO₂ PNPs. Surface-enhanced Raman spectroscopy (SERS) and redox potential data show that the plasmonic Ag⁰@SiO₂ NPs exert a moderate SERS effect on the LMn^II catalyst, and a considerable lowering of the solution redox potential E_h. Our data show that near-field generation is not the sole origin of inhibition of LMn^IV═O formation, while plasmonic heating was insignificant. We suggest that the generation of hot electrons by the Ag⁰@SiO₂ PNPs is implicated, along with near-field generation, in the reversible switch-off of the catalytic process.